1. Field of the Invention
This invention relates to a novel cross-linked polymer having biodegradability. More specifically, the present invention is concerned with a cross-linked, acidic polyamino acid polymer having biodegradability and high water-absorbency. Specifically, this invention pertains to a polymer, which is compatible with the global environment in terms of its biodegradation after its use as a water absorbent in disposable diapers, agricultural or horticultural applications or the like or after its disposal.
2. Description of the Related Art
[Technical Background of Superabsorbent Polymers]
A superabsorbent polymer is a resin capable of absorbing water from several tens of times to several thousands of times as much its own weight, and is used in sanitary products, such as sanitary napkins and disposable diapers, and also in a variety of other fields.
[Related Art on Superabsorbent Polymers]
Known examples of superabsorbent polymers employed in such applications include partial neutralization products of cross-linked polyacrylic acids (JP Kokai No. 55-84304, U.S. Pat. No. 4,625,001), partial hydrolysis products of starch-acrylonitrile copolymers (JP Kokai 46-43995), starch-acrylic acid graft copolymers (JP Kokai 51-125468), hydrolyzation products of vinyl acetate-acrylate ester copolymers (JP Kokai 52-14689), cross-linked copolymers of 2-acrylamido-2-methylpropanesulfonic acid and acrylic acid (EP 0068189), cross-linked polymers of cationic monomers (U.S. Pat. No. 4,906,717), and hydrolysis products of cross-linked isobutylene-maleic anhydride copolymers (U.S. Pat. No. 4,389,513).
These superabsorbent polymers however are accompanied by the problem that they do not degrade following disposal after use.
Under the circumstances, these superabsorbent polymers are currently disposed of by incineration or reclamation. However, it is indicated that disposal in incinerators is a cause of global warming and acid rain in addition to a cause of damage to incinerator materials due to heat occurring during incineration. On the other hand, reclamation disposal is accompanied by problems such as poor stabilization of reclaimed grounds due to the bulky and undegradable nature of plastics and, moreover, is facing a serious problem in that there are no longer many sites suited for reclamation.
Described specifically, these polymers are poor in degradability and remain semipermanently in water or soil. Their disposal presents a very serious problem from the viewpoint of environmental preservation. For example, in the case of polymers for disposable applications, led by sanitary products such as disposable diapers and sanitary napkins, their recycling, if tried, would require substantial expenditure while their incineration, if attempted, would significantly affect the global environment due to the enormous quantities involved. On the other hand, it has been reported that use of a cross-linked polyacrylic acid resin as an agricultural and horticultural water-holding material leads to the formation of complexes with multivalent ions such as Ca.sup.2+ in soil and hence results in the formation of an insoluble layer [Matsumoto et al., KOBUNSHI (High Polymer, Japan), 42, August, 1993]. Such a layer is considered to have low toxicity by itself but is not found at all in the natural world. Nothing is known about their influence on the ecosystem resulting from an accumulation of such polymers in soil over a long period and therefore, a thorough investigation is needed. A cautious attitude towards their use is hence desirable. Likewise, non-ionic resins have a potential problem of accumulating in soil due to their undegradable nature although they do not form complexes. It is therefore likely that they would have adverse effects on the natural world.
Furthermore, these polymerized resins use monomers which are highly toxic to human skin and the like. A great deal of work has been conducted to eliminate such monomers from polymerized products. Nonetheless, their complete elimination is difficult. Still higher difficulties are expected especially in the production on an industrial scale.
[Technical Background of Superabsorbent Polymers Having Biodegradability]
On the other hand, biodegradable polymers have been attracting interest as "globe-compatible materials" in recent years. Their use as super-absorbent polymers has also been proposed.
Known examples of biodegradable superabsorbent polymers employed in such applications include cross-linked polyethylene oxide (JP Kokai 6-157795, etc.), cross-linked polyvinyl alcohol, cross-linked carboxy-methylcellulose (U.S. Pat. No. 4,650,716), cross-linked alginic acid, cross-linked starches, and cross-linked polyamino acids. Among these, the cross-linked polyethylene oxide and cross-linked polyvinyl alcohol have small water absorption and are hence not particularly suited for use as materials in products requiring high water-absorbency such as sanitary products, disposable diapers, disposable dustcloths and paper towels.
Further, these compounds can be biodegraded only by certain particular bacteria, so that under general conditions, their biodegradation will be slow or will not take place at all. Moreover, the biodegradability will be reduced extremely as the molecular weight becomes greater.
In addition, cross-linked saccharides such as cross-linked carboxymethylcellulose, cross-linked alginic acid and cross-linked starches contain many firm hydrogen bonds in their molecules, thereby exhibiting strong interaction between molecules and/or polymers. Accordingly, molecular chains cannot be opened widely meaning that their water-absorbency is not high.
[Technical Background of Polyamino Acid Super-absorbent Polymers]
On the other hand, polymers which are available by cross-linking polyamino acids do have biodegradability and are thus compatible with the global environment. It has also been found that, even when absorbed in the body, they are digested and absorbed by enzymatic action and moreover, they do not exhibit antigenecity in the body and their metabolites are free of toxicity. These polymers are accordingly materials which are also safe for human beings.
As a disclosed example of such a polymer, a process for the production of a polymer having high water-absorbency, which comprises irradiating .gamma. rays to poly-.gamma.-glutamic acid, was reported by Kunioka et al. in KOBUNSHI RONBUNSHU (The Journal of the Society of Polymer Science, Japan), 50(10), 755 (1993). From an industrial viewpoint, however, a .sup.60 Co irradiation system for use in this technology requires considerable equipment for shielding radiation, and sufficient care is also required for its control. This technology is therefore not practical. As a further problem, the high cost of polyglutamic acid as the starting substance can also be mentioned.
In addition, processes for obtaining a hydrogel by cross-linking an acidic amino acid were reported by Akamatsu et al. in U.S. Pat. No. 3,948,863 (corres. JP Kokoku 52-41309) and Iwatsuki et al. in JP Kokai 5-279416. Further, use of cross-linked amino acid polymers as superabsorbent polymers was reported by Sikes et al. in JP PCT Kokai 6-506244 (corres. U.S. Pat. No. 5,247,068 and U.S. Pat. No. 5,284,936), Suzuki et al. in JP Kokai 7-309943 and Harada et al. in JP Kokai 8-59820.
In all the above reports, however, the polymers did not have sufficient water absorbency and were not practically usable.
In the meantime, the present inventors invented superabsorbent polymers having high saline-absorbency as disclosed in JP Kokai 7-224163.
However, a high demand has remained for super-absorbent polymers having high saline-absorbency and excellent water-absorbency, resulting in an outstanding demand for polymers of still higher performance.